Work in progress: user-configured SSH CA authentication for Forgejo

Edit: Made a pull request at https://codeberg.org/forgejo/forgejo/pulls/11746

@tonyg is this WIP in the coding sense, or WIP in the code already does this but you’re setting it up?
@dch I am finishing up the remaining unit/integration tests now. I coded most of it over the weekend. I hope the forgejo team is receptive to an unsolicited feature PR 😅